Silicones have many uses in a variety of fields. They have found large commercial application in products as diverse as sealants, silicone rubbers, adhesives and cosmetics. Silicone oils have been found to be particularly desirable components of cosmetic compositions because the materials impart a dry, smooth uniform feel to the cosmetic composition among other benefits such as increasing apparent luster (or shine). The general use of silicones in cosmetic formulations has been complicated somewhat by the facts that while lower molecular weight silicones impart desirable properties to a composition they are volatile and have low viscosity, while the silicones that overcome these disadvantages are undesirably viscous.
Thus when it has been desirable to utilize low viscosity silicone oils in a cosmetic application, thickening agents have been employed to increase the solution viscosity and slow down the evaporative loss of the volatile low molecular weight silicone oil. This procedure while effective has the disadvantage of decreasing the spreadability of the silicone oil and leaves a heavy greasy feel on the skin. The spreadability and dry smooth feel are properties associated with low viscosity silicone that imparts a desirable feel or hand to the composition when it is applied as a cosmetic formulation. Materials that have found application in attempting to retain the desirable properties of low molecular weight silicone oils in cosmetic compositions while reducing evaporative losses due to high volatility have been among others fatty acid esters of dextrin, fatty acid esters of sucrose, trimethylsilyl substituted polyvinyl alcohols, trimethylsilyl substituted poly saccharides, cellulose ethers containing fatty acid esters, and organically modified clay minerals. These materials have the disadvantage that the light feeling and spreadability imparted by the low viscosity silicone oil is changed with the result that the composition no longer possesses those properties that suggested the use of the low viscosity silicone oil in the first place. Another disadvantage of these thickening agents or volatility inhibitors is that a large number of them are water soluble and must be used as a water dispersions or solutions. With hydrophobic silicone oils the introduction of water thus necessitates the use of emulsifiers and compatibilizers, complicating the formulation of the cosmetic and generally lowering the stability of the formulation with respect to separation of the component phases.
Recently, another approach to retaining the properties of low viscosity silicone oils in cosmetic compositions has been advanced where the low viscosity silicone oil is combined with the addition polymerization product between an organohydrogen polysiloxane and an alkenyl functionalized organopolysiloxane (U.S. Pat. 4,987,169). The organohydrogen polysiloxane utilized in those formulations comprised HSiO.sub.1.5 (T.sup.H), RSiO.sub.1.5 (T), RHSiO (D.sup.H), R.sub.2 SiO (D), R.sub.2 HSiO.sub.0.5 (M.sup.H) and R.sub.3 SiO.sub.0.5 (M) groups. The crosslinking hydride compound utilized was thus a compound of the general formula: M.sub.a M.sup.H.sub.b D.sub.c D.sup.H.sub.d T.sub.e T.sup.H.sub.f. While the cross-linking compound admits T groups either as hydride or substituted by R the preference in this technology is for linear hydride materials because the addition polymerization proceeds more smoothly. The R groups in the above formulas are typical organic substituents known in the art. Subsequently a low molecular weight silicone oil is added to the cross-linked addition polymerized product and the mixture is treated by applying a shearing force. This material may be used by itself as a cosmetic component or as a thickening agent and has the properties of a grease and can be used in a wide variety of industrial lubrication applications as well as the cosmetic application contemplated. The material prepared in this manner can be regarded as a lightly cross-linked elastomer with a volatile, low molecular weight silicone oil dissolved therein. Because the precursor cross-linking hydride is preferably linear and only moderately branched when T groups are incorporated, the addition polymerized product does not possess a tight network of cross-links in the resulting polymer. Linear and lightly crosslinked networks suffer from the disadvantage of having lower efficiency in raising the viscosity of a low molecular weight silicone. In addition to increasing the cost of the product, higher levels of crosslinked silicones result in leaving behind more residue when the volatile, low molecular weight silicone evaporates during use. In some cosmetic applications, e.g. deodorant or antiperspirants, an increased residue is a significant disadvantage as it contributes to staining of the clothing.
Further, linear and lightly crosslinked silicones do not form a film as easily as more tightly crosslinked silicones. The lack of a formation of a film is a disadvantage in a cosmetic application because a film provides a softer, smoother feel as compared to the heavier, less desirable feel of a linear silicone.
For solids, size reduction processes generally result in changing both the average particle size and the particle size distribution. With most solid materials, size reduction techniques usually reduce the average article size and produce a Gaussian distribution of particle sizes. Consequently, the art dealing with size reduction techniques is primarily concerned with controlling the width of the Gaussian distribution, i.e. how broad or how narrow the particle size distribution is, a property typically measured by the width of the distribution peak at half the peak height of the most prevalent particle size. This is typically referred to as a half-width measurement.
Emulsions can also be subjected to size reduction processes with results similar to those obtained for solid processes. An initial particle size and particle size distribution of an immiscible liquid dispersed in a second liquid phase is converted to one having a smaller average particle size. Typically the particle size distribution of the discontinuous phase in an emulsion is best represented by a Gaussian distribution regardless of whether the particle size distribution is measured before or after size reduction.
In contrast to the somewhat well-known predictability of size reduction processes for solids and dispersed liquids, soft solids such as a polymer do not respond in a classical fashion. Unlike emulsions which are insensitive to different methods of applying a shear force to reduce the average particle size, soft polymers frequently yield a random manifold of sizes and shapes. Thus, where it is desirable to reduce the average particle size of an elastomer, it would represent an advance in the art if the result of the size reduction process was controllable, if not predictable.